evaluating whether a string value is an integer

I am just learning php to put to use on building ecommerce sites. I bought a book yesterday to help me on my quest, but it comes up a little short on error handling. From what I understand, any data passed to a php script from an HTML form is of type string. I have managed to make sure people can only order a real number of items (can't enter text when I expect a value) by using the is_numeric() function. However, this does not ensure that people can't order 1.7777777777 hard disks or something silly like that. how do I evaluate a string variable (since I seem not to have other options) to make sure it is of type integer rather than type double? Aaron